Version conflict - UNKNOWN 0.0.0 configparser and flake8 3.6.0

This one is new to me.
I'm trying to build Plone 5.1 with dev packages and getting this error.
Specifically I'm working with the buildout that ships with collective.importexport (
To build against Plone 5.1 I use the command

bin/buildout -c version_plone51.cfg 

this is the error I get:

Error: There is a version conflict.
We already have: UNKNOWN 0.0.0
but flake8 3.6.0 requires 'configparser'.

I ran the identical command a second time and the build worked.

Most likely some obscure bug in zc.buildout, which resulted in a transitorily broken .installed.cfg, which fixed itself on a second run.

Thanks! I've seen zc.buildout do this before. Will tolerate it for now.